Interactions between electrons and the topology of their energy bands can create unusual quantum phases of matter. Most topological electronic phases appear in systems with weak electron–electron interactions. The instances in which topological phases emerge only as a result of strong interactions are rare and mostly limited to those realized in intense magnetic fields1. The discovery of flat electronic bands with topological character in magic-angle twisted bilayer graphene (MATBG) has created a unique opportunity to search for strongly correlated topological phases2,3,4,5,6,7,8,9. Here we introduce a local spectroscopic technique using a scanning tunneling microscope to detect a sequence of topological insulators in MATBG with Chern numbers C = ±1, ±2 and ±3, which form near filling factors of ±3, ±2 and ±1 electrons per moiré unit cell, respectively, and are stabilized by modest magnetic fields. One of the phases detected here (C = +1) was previously observed when the sublattice symmetry of MATBG was intentionally broken by a hexagonal boron nitride substrate, with interactions having a secondary role9. We demonstrate that strong electron–electron interactions alone can produce not only the previously observed phase, but also other unexpected Chern insulating phases in MATBG. The full sequence of phases that we observe can be understood by postulating that strong correlations favour breaking time-reversal symmetry to form Chern insulators that are stabilized by weak magnetic fields. Our findings illustrate that many-body correlations can create topological phases in moiré systems beyond those anticipated from weakly interacting models.
To get the desired quantum effect, the researchers placed two sheets of graphene on top of each other with the top layer twisted at the “magic” angle of 1.1 degrees, which creates a moiré pattern. This diagram shows a scanning tunneling microscope imaging the magic-angle twisted bilayer graphene.
The researchers discovered that the interaction between electrons creates topological insulators: unique devices that whose interiors do not conduct electricity but whose edges allow the continuous and unimpeded movement of electrons. This diagram depicts the different insulating states of the magic-angle graphene, each characterized by an integer called its “Chern number,” which distinguishes between different topological phases.
K. P. Nuckolls, M. Oh, D. Wong, B. Lian, K. Watanabe, T. Taniguchi, B. A. Bernevig and A. Yazdani, “Strongly correlated Chern insulators in magic-angle twisted bi-layer graphene,” Nature 588, 610-615 (2020).
DOI: 10.1038/s41586-020-3028-8.
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